1. Technical Field
The invention relates to an attachment device for fixing an aircraft engine to a strut secured to a structural element of the aircraft, such as a fuselage or wing element.
The attachment device according to the invention can be used on any type of aircraft. A preferred application relates to aircraft of modern design, whose engines are equipped with very large diameter fans.
2. Prior Art
Engines equipping aircraft are either suspended to a strut fixed beneath the aircraft wing, or are laterally attached to a strut fixed to the aircraft fuselage. In both cases the link between the engine and the strut is generally provided by a front attachment device and a rear attachment device. The function of these attachment device is to transmit to the aircraft, by means of the strut, the forces produced by the engine.
To analyze the forces transmitted by the attachment devices, to the engine is allocated an orthonormal fix OXYZ, in which the OX axis, which is directed forwards is oriented in accordance with the longitudinal axis of the engine, the OY axis is oriented laterally in a horizontal plane and the OZ axis is oriented vertically upwards.
With respect to said fix, in the case of an engine suspended on the wing, the forces transmitted to the aircraft structure by the attachment devices are mainly the thrust forces of the engine, essentially applied in accordance with the OX axis, lateral forces due more particularly to a gust of wind, applied substantially in accordance with the OY axis and forces due to the weight of the engine or generated in the case of the aircraft crashing and essentially applied in accordance with the OZ axis. Moreover, attachment devices transit to the aircraft structure a moment along the OX axis and which is due to the rotary movement of the engine. In the case of an engine mounted laterally on the fuselage, the forces applied in accordance with the OY and OZ axes are reversed.
To ensure the fixing of a turbojet engine to an aircraft attachment strut, there are at present two main fixture types, namely the xe2x80x9ccorexe2x80x9d type fixture and the xe2x80x9chybrid fanxe2x80x9d type fixture.
In the core type fixture, the central casing of the turbojet engine is fixed directly to the aircraft strut by a front mount and a rear mount. In this case, the front mount simultaneously absorbs the forces along the OX, OY and OZ axes and the rear mount absorbs the forces along the OY and OZ axes, as well as the moment along the OX axis.
In the case of a hybrid fan-type fixture, a front mount is interposed between the strut and the turbojet engine fan casing and a rear mount is interposed between the strut and the central casing of the turbojet engine, as in the case of the core-type fixture. In this case, the front mount absorbs the forces along the OY and OZ axes and the rear mount absorbs the forces along the OX, OY and OZ axes, as well as the moment along the OX axis. Moreover and as is more particularly illustrated by EP-A-564 126, the absorption of the thrust along the OX axis takes place by means of two rods linking the rear mount to the front part of the central casing of the engine.
Thus, in all conventionally used aircraft engine fixing systems, there is no attachment device able to transmit to the strut both the engine thrust forces exerted along the OX axis and the lateral forces exerted along the OY axis.
As is more particularly illustrated by U.S. Pat. No. 5,275,357, the attachment devices used in existing fixture systems usually comprise an emergency attachment structure. This emergency attachment structure, which is directly connected to the strut or to an intermediate fitting, is solely stressed in the case of a fracture of one or more elements of the main attachment structure. At present, all parts of the emergency attachment structures are passive in normal operation. They do not intervene when the main attachment structure is operational. They only become active during the fracture of one or more parts of the main attachment structure. Thus, their absence would not deteriorate the transmission of forces through the main attachment structure of the attachment device.
Recent use of engines having ever increasing sizes and weights accentuates the problems such as the bending of the engine, vibrations, etc. In order to obviate such problems whilst ensuring a better weight distribution, the replacement of conventional fixing systems by a new system using three instead of two attachment device is being envisaged. In this case, the strut would be connected in front of the central casing of the engine by a front attachment device only taking up forces applied in two orthogonal directions, essentially corresponding to the longitudinal axis OX and the lateral axis OY.
To implement such an attachment device, it is not possible to use the front mount which takes up forces in accordance with the OY and OZ axes in the case of a hybrid fan-type fixture. Thus the plane of the forces to be transmitted is horizontal and no longer vertical.
In addition, the attachment device used for fulfilling this function must preferably be easily dismantlable in order to avoid any time loss during the installation or removal of the engine.
Finally, it is no longer possible to make use of one of the other attachment devices employed in conventional fixing systems, such as the front mount of the core-type fixture, which takes up the forces exerted in accordance with the OX, OY and OZ axes. Thus, due to the fact that the existing attachment devices also take up forces exerted in the OZ direction, their use would make it necessary to bring about more extensive stiffening of the strut in order to ensure a correct taking up of forces transmitted by the attachment device. This would involve adding material and would consequently, in prejudicial manner, increase the weight of the strut. Moreover, this would run counter to an improvement to the performance characteristics of the engine sought by the use of a novel fixing system including a front attachment device only taking up forces mainly applied in accordance with the OX longitudinal axis and the OY axis.
The main object of the invention is a simplified attachment device usable in a novel system for fixing an aircraft engine to a strut, in order to solely ensure the transmission of thrust forces and lateral forces, in a reduced space location and whilst bringing about a minimum disturbance to the aerodynamic air flow.
In secondary manner, a further object of the invention is an attachment device which, if necessary, can be easily dismantled.
According to the invention, this result is obtained by means of a device for the attachment of an aircraft engine to a strut fixed to a structural element of the aircraft, said device comprising a main attachment structure and an emergency attachment structure, characterized in that the main attachment structure only transmits forces exerted in a first direction, mainly parallel to a longitudinal axis of the engine and according to a second direction, mainly parallel to a transverse axis of the engine, perpendicular to the longitudinal axis and to a median plane common to the engine and to the strut, the emergency attachment structure comprising an element fixed to the strut, which also forms part of the main attachment structure.
Bearing in mind the essential function which it fulfils in the attachment device according to the invention, the element common to the main attachment structure and the emergency attachment structure advantageously comprises at least two separate parts able to independently transmit the forces oriented in accordance with the first and the second directions. Thus, if one of the two parts fractures, the forces continue to be transmitted through the other part.
The engine casing and the element common to the main attachment structure and the emergency attachment structure form two adjacent members. In a preferred embodiment of the invention, a first of said members comprises a pin, which projects with clearance into a recess linked with the second member, parallel to a third axis, perpendicular to the longitudinal axis and to the transverse axis of the engine, so as to never be in contact with the walls of the recess when the main attachment structure is operational.
Advantageously, the end of the pin is then normally separated from the bottom of the recess by a distance such that said end can bear against said bottom in order to transmit to the strut forces exerted parallel to the third axis, when the aircraft lands without wheels down.
Preferably, half of the pin is in each of the parts forming the element common to the main attachment structure and the emergency attachment structure.
According to a first embodiment of the invention, the main attachment structure also comprises a crossbar essentially articulated in its centre to said element, two first rods articulated to each of the ends of the crossbar, so as to connect the same to a casing of the engine, mainly in accordance with the first direction, and a third rod directly articulated between said element and the engine casing, substantially in accordance with the second direction.
In this arrangement, the crossbar is preferably articulated to the element common to the main attachment structure and to the emergency attachment structure by a hinge pin mainly parallel to the third axis and located in the median plane.
According to a second embodiment of the invention, the main attachment structure also comprises two rods directly articulated between said element and the engine casing and oriented substantially symmetrically with respect to the median plane, in two intermediate directions between the first direction and the second direction.
In all embodiments, the rods are advantageously articulated by swivel pins. This arrangement ensures that parasitic forces, i.e. oriented in a direction different from the first and second aforementioned force transmission directions, do not pass through the attachment device.
In the case where the engine is a turbojet engine, the attachment device according to the invention is advantageously used for linking the strut with the central casing of said turbojet engine.
Preferably, the element common to the main attachment structure and to the emergency attachment structure comprises a disconnection plane for fixing to the strut by fixing means.